Microscopic surface structures and ORR activities for vacuum-deposited Pt/Ni/Pt(1 1 1) and Pt/Ni/Pt(1 1 0) sandwich structures

Pt0.3–0.6nm/Ni0.3–0.6nm/Pt(1 1 1) and Pt0.3–0.6nm/Ni0.3–0.6nm/Pt(1 1 0) atomic sandwich structures were prepared through alternating vacuum depositions of Ni followed by Pt onto clean Pt(1 1 1) and (1 1 0) substrates at room temperature under ultra-high-vacuum (UHV) conditions. After the samples were transferred from UHV to a 1-atm N2 atmosphere, their oxygen reduction reaction (ORR) activities were evaluated in O2-saturated 0.1 M HClO4 at 0.9 V vs. reversible hydrogen electrode. Pt0.6nm/Ni0.6nm/Pt(1 1 1) and Pt0.6nm/Ni0.6nm/Pt(1 1 0) were most active among the respective Pt/Ni/Pt(1 1 1) and Pt/Ni/Pt(1 1 0) sandwich series: the activities of the former and latter sandwich structures were approximately five- and threefold greater than those of the corresponding clean Pt(1 1 1) and (1 1 0) substrate surfaces. Scanning tunneling microscopy images of the as-prepared Pt0.6nm/Ni0.6nm/Pt(1 1 1) and Pt0.6nm/Ni0.6nm/Pt(1 1 0) surfaces revealed three-dimensionally grown hexagonal-shaped small domains of Pt(1 1 1) (approximately 2 nm in size) and parallelogram-shape (1 1 0) terrace islands oriented along 〈1 1 0〉, respectively. The results indicate that not only the atomic arrangements of the topmost Pt layers but also the nanoscale morphologies of Pt–Ni in the surface vicinities determine the enhancement of the ORR activity of Pt–M alloy catalysts.